1. Field of the Invention
The present invention relates to a water base pigment ink for ink-jet recording based on the use of a pigment as a coloring agent, and an ink-jet recording apparatus which accommodates the same.
2. Description of the Related Art
In the ink-jet recording method, ink droplets are formed by means of a variety of ink discharge methods including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby, and all or a part of the ink droplets are adhered to a recording objective such as paper to perform the recording.
Those known as the ink to be used for the ink-jet recording method as described above include those in which a variety of water-soluble dyes or pigments are dissolved or dispersed in water base or aqueous ink media each comprising water or a combination of water and a water-soluble organic solvent, for which development has been actively made. The water base ink as described above is required to have optimum performance characteristics in order to achieve the object. The requirement is, for example, as follows. That is, neither deposition nor aggregation occurs even when the ink is not used for a long period. No clog-up is caused at the tip of the head and the ink flow passage of the ink-jet printer. The printing quality is satisfactory, and the weather resistance is excellent. Further, for example, neither fungi nor microbes appear in the water base ink and on the image formed on the recording paper. When the dye and the pigment are compared with each other, the pigment is dispersed in the ink medium without being dissolved, while the dye is completely dissolved in the ink medium. Therefore, the dye ink hardly causes the clog-up in the ink flow passage and the tip of the head of the ink-jet printer as compared with the pigment ink. Further, the time-dependent stability of the dye ink is much more excellent than that of the pigment ink. For this reason, the dye ink was dominantly used for the ink for the ink-jet printer at one time. However, in recent years, the pigment such as carbon black has been used for the coloring agent in order to improve the image quality and the weather resistance of the recorded matter. In order to improve the time-dependent stability of the pigment ink, for example, a water base pigment ink, in which the dispersion performance of carbon black is improved by using a surfactant and/or a polymer dispersing agent, has been suggested in Japanese Patent Application Laid-open Nos. 64-60740 and 64-31881. In order to add the antiseptic/fungicidal performance to the ink, an antiseptic/fungicidal agent is generally added in a necessary amount.
However, even in the case of the water base pigment ink dispersed with the surfactant and/or the polymer dispersing agent as described above, the time-dependent stability of such an ink is inferior to that of the dye ink, when the water base pigment ink is compared with the water base dye ink. For example, the following case has been reported. That is, two types of inks, which differed in only coloring agent, i.e., pigment or dye, were prepared. After the preparation of the inks, the both inks were satisfactory in stability of the discharge from the head of an ink-jet printer. However, after the passage of a long period, i.e., after several months, only the water base pigment ink failed in stable discharge from the head of the ink-jet printer. When the water base pigment ink undergoes the passage of a long period, it is considered that the pigment is aggregated to increase the size of pigment particles, and the viscosity is increased, making it difficult to effect the stable discharge from the head of the ink-jet printer. The aggregation of the pigment occurs in different ways depending on the type and the dispersing mechanism of the pigment. In the case of the pigment based on the use of the surfactant and/or the polymer dispersing agent (non-self-dispersing type), even when the dispersion is once in a stable state owing to the adsorption of the surfactant and/or the polymer dispersing agent to the pigment surface, the surfactant and/or the polymer dispersing agent on the pigment surface is gradually liberated during the storage for a long period, or the action thereof is inhibited to lower the dispersion stability. Also in the case of the pigment of the self-dispersing type in which the pigment is spontaneously dispersed owing to the reforming treatment for the pigment surface without using the surfactant and/or the polymer dispersing agent, it has been confirmed that the action thereof is inhibited, and the dispersion stability is lowered.
The present invention has been made in order to solve the problems as described above, an object of which is to provide a water base pigment ink for ink-jet recording in which no pigment is aggregated even when the ink is left to stand for a long period, neither fungi nor microbes or the like appear, the clog-up scarcely occurs in an ink flow passage and a tip of a head of an ink-jet printer, and the storage stability is excellent, and a recording apparatus based on the use of the ink.
According to a first aspect of the present invention there is provided an ink for ink-jet recording comprising water; and a pigment; wherein a total amount of free monovalent metal is not more than 300 ppm in the ink, and a total amount of free divalent or polyvalent metal is not more than 5 ppm in the ink. As a result of diligent studies in order to solve the problem of the decrease in dispersion stability of the pigment as described in the section of the related art, it has been found out that the dispersion stability is affected by the amount of free metal ion in the water base pigment ink. Further, it has been also found out that even when the metal element is present at the inside of the pigment or on the surface of the pigment, the metal element does not affect the time-dependent stability of the ink, provided that the metal element is not liberated and ionized. The free metal ion functions to neutralize the negative electric charge generated on the pigment surface. Especially, in the case of the pigment of the self-dispersing type, if the free metal ion exist in an amount of not less than a certain amount, then the function to neutralize the electric charge on the pigment surface is enhanced, the repulsion exerted between the pigments is decreased, and the dispersion stability is lowered. Also in the case of the pigment of the non-self-dispersing type, even when the dispersion is once in a stable state owing to the adsorption of the surfactant and/or the polymer dispersing agent to the pigment surface, if the free metal ion exists in an amount of not less than a certain amount, then the surfactant and/or the dispersing agent on the pigment surface is gradually substituted with the metal ion, and the dispersion stability is lowered.
The ink of the present invention may further comprise an antiseptic/fungicidal agent. Many additives, which are commercially available as the antiseptic/fungicidal agent, contain metal ions. However, in the present invention, the amount of the metal ion in the ink is previously restricted as described above. Therefore, even when the antiseptic/fungicidal agent is added, the amount of the metal ion in the ink is not so excessive as to deteriorate the dispersion stability. Accordingly, in the present invention, it is possible to add a sufficient amount of the antiseptic/fungicidal agent, and thus the antiseptic/fungicidal performance of the ink is enhanced.
According to a second aspect of the present invention, there is provided an ink-jet recording apparatus comprising an ink-jet head; an ink tank which accommodates an ink to be supplied to the ink-jet head; and the ink of the present invention which is accommodated in the ink tank. The ink-jet recording apparatus of the present invention is based on the use of the ink of the present invention which is excellent in dispersion stability of the pigment. Therefore, no clog-up occurs in the ink flow passage and the nozzle of the head. It is possible to stably discharge the ink over a long period. The ink tank may be an ink container fixedly provided in the ink-jet recording apparatus or an ink cartridge which is replaceable.